In deep oil wells or oil wells in severe corrosive environments involving corrosive substances such as humid carbon dioxide gas (CO2), hydrogen sulfide (H2S) or chloride ion (Cl−), austenite-ferrite duplex stainless steel pipes having a large content of Cr such as 22Cr steel or 25Cr steel are used as oil well pipes.
These austenite-ferrite duplex stainless steels as having been subjected to the solution treatment usually applied in the production thereof can attain at most such a strength that tensile strength (TS) is of the grade of 80 kgf/mm2 (785 MPa) and yield strength (0.2% yield stress) is of the grade of 60 kgf/mm2 (588 MPa). In consideration of this issue, Patent Document 1 proposes a method for obtaining a high-strength duplex stainless steel pipe that contains 0.1 to 0.3% of N is subjected to a cold working with a reduction of area of 5 to 50%, and thereafter the pipe is heated at 100 to 350° C. for 30 minutes or more to yield the desired pipe. In this case, it is claimed that a duplex stainless steel pipe having a high strength is obtained by combining the work hardening due to the cold working with the aging treatment.
However, in these years, oil wells have a remarkable tendency toward being deeper, and hence, for the purpose of the use in environments severer than those hitherto experienced, it is required to produce duplex stainless steel pipes which are high in strength, in particular, of the grade of 110 to 140 ksi (the minimum yield strength is 758.3 to 965.2 MPa) and additionally have various strength levels defined in the specifications. Thus, for that purpose, it is not sufficient to consider only the content of N as in Patent Document 1, but it is also necessary to consider the contents of the other components, and additionally it is necessary to more strictly control the cold working ratio. The production method disclosed in Patent Document 1 offers a problem of the production efficiency deterioration or the cost increase due to the addition of the aging treatment step.
For the purpose of attaining a high corrosion resistance and a high strength, Patent Document 2 discloses a method in which a Cu-containing duplex stainless steel material is subjected to a cold working with a reduction of area of 35% or more, thereafter heated and quenched, and then subjected to a warm working. This document discloses a conventional example, wherein a Cu-containing duplex stainless steel wire rod is subjected to a solid-solution heat treatment and thereafter subjected to a cold working with a reduction of area of 25 to 70%, and thus a high-strength wire rod having a tensile strength of 110 to 140 kgf/mm2 has been obtained. However, this discloses only an increase of the tensile strength due to the cold working in relation to a wire rod but not with a pipe, and hence it is not clear what is the level of the yield strength significant in the material design of the oil well pipes.
Further, Patent Document 3 describes a high strength steel that can be attained by a low-reduction cold working based on forging. However, here is merely disclosed a method for improving the strength by successively forging with a cold working ratio of about 0.5 to 1.6% over the whole region, in the longitudinal direction, of a duplex stainless steel stock that has been subjected to a solution treatment while the stock is being imparted with rotation.
[Citation List]
[Patent Documents]
[Patent Document 1] JP2-290920A
[Patent Document 2] JP7-207337A
[Patent Document 3] JP5-277611A